A large number of electromagnetic signal sources are located in populated areas, such as Wireless Access Points (WAPs) of Wi-Fi networks, mobile phone towers, Bluetooth® beacons, and so on. Wireless technology-based positioning systems, and Wi-Fi positioning systems in particular, can provide an estimate of a user's position based on properties of signals received from such signal sources, such as signal strength, and further information, typically stored in a database, that provides a location for each signal source.
Such electromagnetic signal sources, and WAPs in particular, are often not uniformly distributed. Often WAPs may be clustered in particular buildings and/or on particular floors, which in practice can reduce the accuracy of the systems.
FIGS. 1A and 1B illustrate one aspect of this problem. In FIG. 1A, a user device 102 can detect signals from a single WAP 104 in one building and a cluster of WAPs 106, 108, 110 in another building. One of the building walls 114 may be relatively thin and/or permeable to Wi-Fi signals, and the other wall 116 may be relatively thick and/or impermeable, for example. The attenuation of the Wi-Fi signals from the WAPs 104, 106, 108, 110 can be computed and used to estimate the distance between the user terminal and the WAPs, which can in turn be used to estimate the position of the user terminal 102. FIG. 1B illustrates how the geometry of the WAPs may appear from the point of view of the position estimation software. In this case the length L2 may appear to be greatly exaggerated compared to the length L1, for example, due to locally high attenuation of the signals from the WAPs 106, 108, 110 in the vicinity of the user terminal 102. Various techniques can be used to refine the position estimates based on assumptions and/or prior knowledge of the environment in the vicinity of the user terminal 102, and signal propagation effects can to a degree cancel out, but in a case such as this the clustering of WAPs 106, 108, 110 can make the position estimate more susceptible to systematic errors (such as a locally thicker stretch of wall and the like) or can otherwise skew the position estimate due to a highly non-uniform distribution of signal sources around the user terminal 102.
Further problems arise in that the accuracy of data in positioning system databases can vary considerably, and the amount of data that is required to be stored in order to provide properly calibrated positioning data for all detectable WAPs and other signal sources can impose a serious burden in terms of bandwidth, storage capacity and the processing power that is required to generate position estimates. The present invention seeks to address these and other problems in the prior art.